Multilevel inverters are widely used in high-power high-voltage applications due to advantageous performance over two-level inverters, including reduced voltage pressure or tension on the power devices, lower harmonics, lower instantaneous rate of voltage change (dv/dt), and lower common-mode voltage. However, the inherent voltage drift of the DC-link capacitors of the multilevel inverters will degrade the performance of the multilevel inverters, in terms of higher voltage pressure on the power devices, higher harmonics, higher electromagnetic interference, and so on. If the voltage drift of the DC-link capacitors is not limited during the operation of the multilevel inverter, then the unbalances of DC-link capacitor voltages even lead to the collapse of some of these voltages under a wide range of operating conditions.
Several approaches have been suggested to balance the DC-link capacitor voltages of the multilevel inverters. One approach is realized by introducing extra circuits to keep the DC-link voltages balanced such as by A. Jouanne, et al., “A multilevel inverter approach providing DC-link balancing, ride-through enhancement, and common-mode voltage elimination,” IEEE Trans. Ind. Electron., vol. 49, no. 4, pp. 739-745, August 2002. However, A. Jouanne's method requires additional power hardware, which adds to the cost and complexity of the system. In another approach, the DC-link voltage balancing is achieved with the help of another active power circuit which is already part of the system as by A. Yazdani, et al., “Dynamic model and control of the NPC based back-to-back HVDC system,” IEEE Trans. Power Delivery, vol. 21, no. 1, pp. 414-424, January 2006. However, A. Yazdani's method is proper for a double converter back-to-back application, but not suitable for a stand-alone multilevel inverter.
The third approach for DC-link voltage balancing control for multilevel inverters is to modify the switching pattern of the inverter according to a control strategy to balance the DC-link capacitor voltages, which attracts more and more attention nowadays because no additional hardware is needed. Space vector pulse width modulation, also called SVPWM, is the most attractive modulation strategy for multilevel inverters because SVPWM provides significant flexibility for optimizing switching waveforms, and because SVPWM is well suitable for digital signal processor implementation. One virtual-space-vector pulse width modulation based DC-link voltage balancing control method is introduced by S. Busquets, et al., “Pulsewidth Modulations for the Comprehensive Capacitor Voltage Balance of n-Level Three-Leg Diode Clamped Converters, IEEE Trans. Power Electron., vol. 24, no. 5, pp. 1364-1375, 2009. In S. Busquets's method, however, the addition of the three phase currents is required to be zero, which limits the application of the method, and the complexity of the method will be increased dramatically with the increase of the inverter level. Some other SVPWM DC-link voltage balancing schemes based on objective function optimization can be found in M. Saeedifard, et al., “Analysis and Control of DC-Capacitor-Voltage-Drift Phenomenon of a Passive Front-End Five-Level Converter”, IEEE Trans. Ind. Electron., vol. 54, no. 6, pp. 3255-3266, 2007; and L. Su, et al., “A Novel DC Voltage Balancing Scheme of Five-Level Converters Based on Reference-Decomposition SVPWM,” Applied Power Electronics Conference and Exposition, pp. 1597-1603, Orlando, February 2012. In both M. Saeedifard's and L. Su's methods, the addition of the instantaneous currents of the DC-link capacitors are assumed to be zero, which is not accurate when the voltage of the DC source has fluctuation or the capacitances of the DC-link capacitors are not strictly equal. Moreover, the duty cycles of the space vectors are fixed in M. Saeedifard's and L. Su's methods, which can't provide the best control effect for all operation conditions. In addition, in M. Saeedifard's method, the objective function needs to be differentiable, which makes M. Saeedifard's method ineffective when nondifferentiable objective functions are adopted.
Accordingly, there is a need for general DC-link voltage balancing method for multilevel inverters.